The typical cell structure of a solar cell is comprised of a layer structure that includes a positive electrode, a mixture of organic semiconductors and a negative electrode. The material used for the positive electrode can, for example, be ITO/PEDOT:PSS. The semiconductor mixture is composed of an organic material, for example a polymer. The semiconductor mixture includes both n-conductive and p-conductive semiconductor molecules. This mixed semiconductor layer is known as a bulk heterojunction layer. Additional materials that are used concern the negative electrode, for example, which can be made of Ca/Ag or LiF/Al. The above-listed substances do not, however, exclusively constitute the elements concerned, but rather, other material combinations are also feasible. The donor present in the bulk mixed layer can for example be a conjugated polymer, and the acceptor can for example be a soluble methanofullerene.
A major difficulty in the production of bulk heterojunction solar cells is to create a desirable phase morphology with respect to the n-semiconductor and the p-semiconductor. This problem is due, among other things, to the different solubility of the individual components in the common solvent.
Attempts have heretofore been made to obtain the desired morphology using only one solvent, which dissolves the organic semiconductors used in a way that permits the fabrication of suitably thick and homogeneous, well-blended semiconductor films.
The choice of solvents is very limited, owing to the particularly high requirements imposed on them. For example, it is desirable for the formation of a concentration gradient in the distribution of the semiconductors in the so-called bulk to be accompanied by a suitable morphology for the semiconductor mixture in the applied film. The choice of solvent is also particularly limited, for example, by phase separation within the range of the exciton diffusion length [1].
Alternatively, geometries created by the successive application of the individual semiconductor layers have heretofore been investigated. Such attempts have included the use of a so-called bilayer with a sharp interface between the two semiconductors [2]. A “stratified multilayer” has also been developed [3]. This involves the formation of interdiffusion layers in which the upper layer is able to penetrate slightly into the lower layer. This leads to partial interleaving or mixing by diffusion.